Single-layer and multi-layer structures for integrated silicon photonics optical gyroscopes

ABSTRACT

Disclosed herein are configurations and methods to produce very low loss waveguide structures, which can be single-layer or multi-layer. These waveguide structures can be used as a sensing component of a small-footprint integrated optical gyroscope. By using pure fused silica substrates as both top and bottom cladding around a SiN waveguide core, the propagation loss can be well below 0.1 db/meter. Low-loss waveguide-based gyro coils may be patterned in the shape of a spiral (circular or rectangular or any other shape), that may be distributed among one or more of vertical planes to increase the length of the optical path while avoiding the increased loss caused by intersecting waveguides in the state-of-the-art designs. Low-loss adiabatic tapers may be used for a coil formed in a single layer where an output waveguide crosses the turns of the spiraling coil.

RELATED APPLICATIONS

This application is a divisional of and claims the benefit of U.S.patent application Ser. No. 16/894,120, filed Jun. 5, 2020, titled“Single-Layer and Multi-Layer Structures for Integrated SiliconPhotonics Optical Gyroscopes,” which claims the benefit of U.S.Provisional Patent Application Nos. 62/986,379, filed Mar. 6, 2020,titled “Process Flow for Fabricating Integrated Photonics OpticalGyroscopes,” and 62/896,365, filed Sep. 5, 2019, titled, “Single-Layerand Multi-Layer Structures for Integrated Silicon Photonics OpticalGyroscopes,” and 62/858,588, filed Jun. 7, 2019, titled “IntegratedSilicon Photonics Optical Gyroscope On Fused Silica Platform,” theentireties of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to low-loss optical waveguidestructures, and specifically to use of the waveguide structures forintegrated photonics-based optical gyroscopes.

BACKGROUND

Gyroscopes (also referred to in short as “gyros”) are devices that cansense angular velocity. The applications of gyroscopes include, but arenot limited to, military, aircraft navigation, robotics, autonomousvehicles, virtual reality, augmented reality, gaming etc. Gyroscopes canbe mechanical or optical, and can vary in precision, performance, costand size. Since optical gyroscopes do not have any moving parts, theyhave advantages over mechanical gyroscopes as they can withstand effectsof shock, vibration and temperature variation better than the mechanicalgyroscopes with moving parts. The most common optical gyroscope is thefiber optical gyroscope (FOG) that operates based on interferometricmeasurements of optical phase shift due to the Sagnac effect (aphenomenon encountered in interferometry that is elicited by rotation).Construction of a FOG typically involves a coil comprising several turnsof polarization-maintaining (PM) fiber. Laser light is launched intoboth ends of the PM fiber coil so that two optical beams travel inopposite directions. If the fiber coil is moving, the optical beamstraveling in opposite directions experience different optical pathlengths with respect to each other. By setting up an interferometricsystem, one can measure the small path length difference that isproportional to the area of the loop enclosed by the turns of the fibercoil and the angular velocity of the rotating fiber coil. This pathlength difference is expressed as a phase signal.

Phase signal of an optical gyro is proportional to the Sagnac effecttimes the angular rotation velocity, as shown in the following equation:

Δϕ=(8πNA/λc)Ω

where,

N=number of turns in the gyro,

A=area enclosed

Ω=angular rotation velocity

Δϕ=optical phase difference signal

λ=wavelength of light

c=speed of light

Fiber-based gyroscopes can provide very high precision, but at the sametime, they are of larger footprint, are very expensive, and are hard toassemble due to the devices being built based on discrete opticalcomponents that need to be aligned precisely. Often, manual alignment isinvolved, which is hard to scale up for volume production.

The key to FOG's performance is the long length of high quality, lowloss, optical fiber that is used to measure the Sagnac effect. Thepresent inventors recognize that with the advent of integrated siliconphotonics suitable for wafer scale processing, there is an opportunityto replace FOGs with smaller integrated photonic chip solutions withoutsacrificing performance.

SUMMARY

Present inventors propose using waveguide based integrated photonicscomponents instead of fibers for cost-effective easy integration on asemiconductor platform which is much more promising for volumeproduction of gyroscopes. This application describes varioussingle-layer and multi-layer structures and processes for fabricatingsilicon nitride (SiN) waveguide cores in a silicon fab, as elaboratedbelow. Note that the word “layer” as used in single-layer andmulti-layer in the specification means a section of the waveguidestructure with a waveguide core surrounded by corresponding claddings.The term “plane” has been used interchangeably with the term “layer” toconvey the same concept of a section with a waveguide core surrounded bycorresponding claddings.

Photonics based optical gyros have reduced size, weight, power and cost,but in addition can be mass produced in high volume, are immune tovibration and have the potential to offer performances equivalent toFOGs.

One key element of this integrated photonic solution is to produce verylow loss waveguides that can be manufactured using wafer scale processesand can be used to replace the long length PM optical fiber in opticalgyros. The technology platform used for this integrated photonics basedoptical gyros is based on silicon nitride (Si₃N₄)—sometimes alsoreferred to as SiN for simplicity.

Disclosed herein are configurations and methods to produce very low losswaveguides that can be used as integral component for a small-footprintintegrated optical gyroscope, which is abbreviated as SiPhOG™ (SiliconPhotonics Optical Gyroscope), though compound semiconductor (III-Vsemiconductor) based integrated photonics optical gyroscopes are alsowithin the scope of this disclosure. Furthermore, some embodiments ofthe integrated photonics optical gyroscopes may have a combination ofsilicon photonics and III-V semiconductor based photonics components.

An optical gyroscope module has a front-end chip with electrical andoptical components for optical signal processing and a waveguide chipcoupled to the front-end chip. In the waveguide chip (also referred toas the “gyro chip” or “sensing chip”), low-loss waveguide core may bemade of silicon nitride (Si₃N₄), and the waveguide cladding may be madeof fused silica or oxide. The oxide layer can be grown if the substratehas a crystalline Si structure to support growth of oxide.Alternatively, the oxide layer can be deposited, such as using precursorlike Tetraethyline Orthosilicate (TEOS), or other precursors describedelsewhere in this specification. For non-crystalline fused silicasubstrate, deposition is the method of forming an oxide layer. Thiswaveguide structure is also referred to simply as SiN waveguide, and achip containing the SiN waveguide may be referred to as a SiN waveguidechip in the figures. As will be discussed below, the waveguide structurecan be single layer or multi-layer.

By using pure fused silica (non-crystalline glass, which is referred tosimply as “glass” at some places in the specification) substrates asboth top and bottom cladding of the SiN waveguide, the propagation losscan be well below 0.1 db/meter. The solution involves patterning alow-loss waveguide-based gyro coil (patterned in the shape of a spiralwith several turns), that may be distributed among one or more verticalplanes to increase the length of the optical path while avoiding theincreased loss caused by intersecting waveguides in the state-of-the-artdesigns. Additionally, the present inventors recognize that instead of aspiral, a simple ring resonator or other geometries may be used asSiPhOG. For Ring Resonator (RR) design the optical signal is coupledinto the ring waveguide via a straight waveguide disposed at a couplingdistance away from the ring waveguide on the same plane or on adifferent plane. Note that in the specification and claims, the term“loop” is meant to encompass both a single turn-loop (such as in ringresonator) or a multi-turn loop (such as in a fiber coil).

More specifically, in one aspect, a waveguide structure is disclosed,comprising: a first fused silica wafer with a first etched trenchthereon, wherein the first fused silica wafer acts as a first cladding;a first core formed in the first etched trench of the first fused silicawafer, wherein the first core comprises silicon nitride (SiN); and, asecond fused silica wafer bonded to the first fused silica wafer,wherein the second fused silica wafer acts as a second cladding, whereinthe second cladding is adjacent to the first cladding, and wherein thefirst cladding and the second cladding collectively completely surroundthe first core from the top, bottom and sides.

In another aspect, a waveguide structure is disclosure, comprising: afirst fused silica wafer with a first etched trench thereon, wherein thefirst fused silica wafer acts as a first cladding; a first core formedin the first etched trench of the first fused silica wafer, wherein thefirst core comprises silicon nitride (SiN); an interposing layer thatseparates two planes of the waveguide structure; a second fused silicawafer with a second trench thereon, wherein the second fused silicawafer acts as a second cladding; and, a second core formed in the secondetched trench of the second fused silica wafer, wherein the second corecomprises silicon nitride (SiN). The first fused silica wafer with thefirst core is bonded to a first surface of the interposing layer, andthe second fused silica wafer with the second core is bonded to a secondsurface of the interposing layer, wherein the first surface and thesecond surface of the interposing layer are opposite to one another.

In yet another aspect, a waveguide structure is disclosed, comprising: afirst fused silica wafer acting as a first cladding; a patterned siliconnitride (SiN) layer formed on top of the first fused silica wafer,wherein the patterned SiN layer acts as a first core; an oxide layercoinciding with the patterned SiN layer, wherein the oxide layersurrounds the first core from the sides; and, a second fused silicawafer bonded to the oxide layer and the patterned SiN layer, wherein thesecond fused silica wafer acts as a second cladding, and wherein thefirst core is surrounded by the first fused silica wafer on the bottom,and second fused silica wafer on the top and the oxide layer on thesides.

While a focus of this patent application is waveguide fabrication andthe fused silica material used above and below the SiN waveguide core,the inventors design the entire photonics optical gyroscope chip(including a front-end chip and the waveguide chip) with higher-levelsystem architecture and key performance parameters in mind, including,but not limited to laser performance, tuning parameters, detectorparameters, as well as packaging considerations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousimplementations of the disclosure.

FIG. 1A is a schematic of a SiN waveguide (cross sectional view) onconventional silicon substrate with asymmetric upper cladding (e.g.,deposited oxide like TEOS) and lower cladding (e.g., frown oxide),according to embodiments of the present disclosure.

FIG. 1B is schematic of a SiN waveguide (cross sectional view) onsilicon substrate, which is bonded to another silicon substrate for asymmetrical upper and lower cladding), according to embodiments of thepresent disclosure.

FIG. 2 is a schematic of a SiN waveguide (cross sectional view) which isbonded to a fused silica (glass) substrate acting as the uppercladding), according to embodiments of the present disclosure.

FIG. 3 is a schematic of a SiN waveguide (cross sectional view) on afused silica substrate which is bonded to another fused silica (glass)substrate for the upper cladding), according to embodiments of thepresent disclosure.

FIG. 4 is schematic of a SiN waveguide (cross sectional view) where atrench is first etched into a fused silica substrate and then the SiN isdeposited to fill the trench and then the wafer is polished usingchemical mechanical polishing (CMP), according to embodiments of thepresent disclosure. After CMP, the wafer is bonded to another fusedsilica (glass) substrate without an intermediate oxide layer. Thisprovides a low loss defect free cladding that is symmetric on both sidesof the SiN.

FIG. 5 is schematic of a multi-layer stacked SiN waveguide (crosssectional view) structure, where the waveguides are created (per theprocess described for the structure in FIG. 4) into two fused silicasubstrates, which are then bonded together, separated by an additionalinterposing layer, according to embodiments of the present disclosure.This additional interposing layer could be TEOS, oxide, or could beanother fused silica layer.

FIG. 6 is an exploded perspective view of a spiral waveguide basedSiPhOG), according to embodiments of the present disclosure, where theoutput waveguide does not intersect with the turns of the gyro coil,according to embodiments of the present disclosure. There are portionsof the gyro coil both on the top plane and the bottom plane, and theoutput waveguide comes out from the same plane as the input waveguide.

FIG. 7 is a schematic of a spiral waveguide based SiPhOG (crosssectional view) structure, according to embodiments of the presentdisclosure where waveguides in at least one layer are etched andpatterned into a fused silica substrate, which is then bonded togetherwith another layer of waveguide, separated by a vertical couplingdistance, both the layers of waveguides forming part of the same gyrocoil.

FIG. 8 shows a first version of ring resonator-based SiPhOG structurecreated in fused silica substrate per FIG. 4, according to embodimentsof the present disclosure.

FIG. 9 shows a second version of ring resonator-based SiPhOG structurecreated in fused silica substrate per FIG. 4, according to embodimentsof the present disclosure.

FIG. 10 shows the top view of a rectangular gyro coil in one layer withsymmetrical pitch between the waveguides everywhere, according toembodiments of the present disclosure.

FIG. 11 shows the top view of a rectangular gyro coil in one layer withasymmetrical pitch between the waveguides on the side where the outputwaveguide crosses the waveguides of the coil, according to embodimentsof the present disclosure.

FIG. 12 shows a blow-up of the crossing region in the structure shown inFIG. 11, showing adiabatic tapers to lower loss, according toembodiments of the present disclosure.

FIG. 13 is an exploded perspective view of a rectangular gyro coil wherethe output waveguide does not intersect with the waveguides of the coil,according to embodiments of the present disclosure. In this embodiment,the output waveguide comes out from a different plane as the inputwaveguide.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed to configurations ofcompact ultra-low loss integrated photonics-based waveguides for opticalgyroscope applications, and the methods of fabricating those waveguideson a fused silica platform for ease of large scale manufacturing. Thewaveguides have a propagation loss of 0.1 db/m or even lower. Thewaveguides are part of a SiPhOG chip, which may be part of an inertialmeasurement unit (IMU) package. Note that IMU may have other components,such as accelerometers, in addition to the SiPhOG chip. Therefore,making the SiPhOG part compact reduces the overall size, weight powerand cost of the IMU. This weight reduction can be crucial for certainapplications, for example, lightweight unmanned aerial vehicles. IMU maybe a needed technology component a for more establishing sensingtechnologies for autonomous vehicles, such as LiDAR (Light Detection andRanging), radar and cameras that will be used in future generation ofautonomous vehicles.

FIG. 1A is a schematic cross-sectional view of a SiN waveguidefabricated on a conventional silicon substrate with asymmetric uppercladding (e.g., TEOS) and lower cladding (grown oxide) around the SiNwaveguide. A non-limiting illustrative dimension of the SiN waveguide isa height (i.e. thickness ‘h1’ of the waveguide layer) of 90 nm and alateral width (V) of 2.8 μm. Persons skilled in the art would appreciatethat these illustrative dimensional values mentioned in thespecification are not limiting to the scope of the disclosure.TetraEthyl OrthoSilicate (TEOS) is a commonly used precursor to silicondioxide in the semiconductor industry. In the embodiment shown in FIG.1A, the silicon substrate (125) has pre-grown oxide layers (120 and 130)on both sides. An example thickness of the oxide layers is 15 μm. Inother embodiments, only the top oxide layer 130 may be grown on thesilicon substrate 125. The SiN waveguide core 140 is then formed andphotolithographically or otherwise patterned on the oxide layer 130. Anoxide layer (135), such as a TEOS layer, is then deposited to cover theSiN waveguide. The deposited oxide layer may initially have a thicknessof 3 μm. The hydrogen in the oxide layer needs to be minimized, and thedeposition rate is adjusted accordingly. The hydrogen in the depositedTEOS layer is minimized by controlling the deposition parameters, suchas pressure, gas sources, temperature, Nitrogen gas in the furnace etc.As an alternative to silane-based precursor for TEOS deposited by LowPressure Chemical Vapor Deposition (LPCVD), one can also usechlorine-based source (SiCl₂H₂) for oxide deposition or deuteratedsource (SiD₄) to further reduce the amount of hydrogen in the depositedoxide layer. These alternative methods reduce the need to drive outhydrogen and other impurities by thermal annealing. Similarly, thoughSiN can be deposited by LPCVD, an alternative is to deposit SiN usingdeuterated source SiN:D that reduces N—H bonds and hence reducesabsorption near the wavelengths of interest.

The wafers may need to be annealed for a period of time (e.g., theperiod be 2-10 hours) at elevated temperature (e.g., 1100-1300° C.) tohelp diffuse out the hydrogen to reduce the loss. Total final thicknessof the TEOS (or oxide) layer may be obtained in several rounds, e.g.each round producing a sub-layer that is 0.1 to 0.2 μm thick. Note thatthe structure 100 shown in FIG. 1A has asymmetric cladding around thewaveguide, as the upper cladding is deposited TEOS, and the lowercladding is grown oxide. The waveguide loss depends largely on thequality of the TEOS layer acting as the upper cladding.

FIG. 1B shows an alternative structure 110 where the upper and lowercladdings are more symmetric. This structure is obtained via waferbonding. The initial thickness of the deposited oxide is reduced by ‘h2’(by chemical mechanical polishing (CMP) or other means) so that the topof the wafer is substantially planar and flush with the top of the SiNwaveguide. Then another wafer with a silicon layer 150 with top andbottom pre-grown oxide layers 155 and 145 is wafer-bonded to the planartop surface of the polished wafer with the SiN waveguide core 140. Thethickness of the waveguide SiN core 140 may be 60-90 nm. Note that theloss value in this configuration depends on the quality of CMP inaddition to the quality of the deposited oxide and the time andtemperature of the anneal.

FIG. 2 shows a structure 200 where the upper cladding is a glass wafer(210), substituting the oxide-coated silicon wafer 150 shown in FIG. 1B.The material of the glass wafer may be non-crystalline fused silicainstead of silicon with pre-grown oxide. Using IR-pure fused silicainsures that it is impurity free and has low absorption in the infra-red(IR) wavelength region (1500-1600 nm).

FIG. 3 shows a structure 300 where the upper cladding and lower claddingboth may be glass wafers (310 and 320). Though FIG. 3 shows an oxidelayer 135 (such as a TEOS layer) in between the glass wafers 310 and320, the oxide layer 135 may not be needed if the SiN waveguide core 140is etched into the bottom wafer 310 in an all-glass configuration asshown in FIG. 4. In some embodiment, instead of having the oxidelaterally encompassing the core 140, layer 135 may have air gapimmediately adjacent to the core 140. For structural support, the oxidelayer may coincide with the air gap layer.

FIG. 4 shows an example of fabricating an all-glass configuration 400.The process starts with a glass substrate 410. Then a trench for the SiNwaveguide core 140 is etched into the first glass substrate 410, asshown in FIG. 4. Then SiN is deposited to fill the trench and then CMPis performed to planarize the top surface of the wafer 410 with the SiNwaveguide core 140 etched into it. Then a second glass wafer 420 may bebonded on top of the wafer 410 having the SiN waveguide core 140. Waferbonding process can be done at various temperature and pressure based onthe integration flow and considerations and the materials involved. Forexample, a covalent bonding for fused silica wafers is done byactivation of the two surfaces that are bonded by oxygen plasma followedby a thermo-mechanical step that create the final bond. Anodic bondingis also an alternative approach that can be used to bond the two fusedsilica wafers. Also, bonding can be done between an oxide layer and afused silica wafer.

FIG. 5 illustrates the concept of stacked waveguides in two planes in astructure 500. Waveguide cores 540 a and 540 b have a finite verticalgap (equal to the thickness of the layer 535) in between them. As willbe described later with respect to subsequent figures, the vertical gapfacilitates avoiding crossing the waveguides on the same plane (whichincreases overall loss), while still making optical coupling possiblebetween the layers. A glass wafer may be polished down to 3-5 μm to actas an intermediate layer 535. A glass wafer 510 may have an etchedwaveguide core 540 a. The polished glass wafer with intermediate layer535 is bonded to the wafer 510. Another glass wafer 520 (similar towafer 510) with an etched waveguide core 540 b may be flipped on top ofthe intermediate layer 535. The all-fused silica (“all-glass”)configuration offers pure symmetric cladding around the waveguide coresand results in the lowest loss. Note that the intermediate layer beingglass 535 is just one configuration. In alternative embodiments, thelayer 535 may be an oxide layer or made of some other materials.

Persons skilled in the art would appreciate that though FIG. 5 showsonly two layers stacked on top of each other, the process can berepeated to add more waveguide layers in the vertical direction. Thescope of this disclosure encompasses tuning the process parameters, suchas the SiN waveguide thickness, lithography, etch, CMP etc. to ensurethe lowest possible loss. For example, using poor quality oxidedeposited by chemical vapor deposition (CVD) increases the lossmanifold. Additionally, the process can be tuned to control sidewallroughness for the waveguides, as that can increase the overall loss aswell. It might be difficult to decouple waveguide propagation loss withloss caused by line edge roughness (LER) or line width roughness (LWR).In general, by conducting rigorous experiment, the inventors have comeup with a process involving fused silica substrate, deposited cladding,CMP, wafer bonding (layer transfer), optimized thickness of the SiNwaveguide, and high-temperature anneal, whose combined effect is toachieve ultra-low-loss (<0.1 db/m) waveguides.

FIG. 6 is an exploded perspective view of a spiral waveguide basedSiPhOG 600 where the output waveguide does not intersect with the turnsof the gyro coil. There are portions of the same gyro coil both on thetop plane and the bottom plane, and the output waveguide comes out fromthe same plane as the input waveguide for efficient coupling. Personsskilled in the art would appreciate that efficient coupling withexternal components (e.g., lasers, detectors etc.) on the front-end chipmay depend on the on the output waveguide and the input waveguide to beon the same plane. Also, by distributing the total length of the coilbetween two layers (top and bottom), the length of propagation can beincreased without increasing the footprint of the gyro coil, butintersection of waveguides can be avoided. Conventional photonic gyrosencounter the problem of intersecting waveguides, as the direction ofpropagation of light has to remain the same within the coil.Intersecting waveguides lead to increased loss, which the design in FIG.6 with non-intersecting waveguides in two or more layers can avoid.

In FIG. 6, the substrate 620 is fused silica or Si substrate. Layers 610and 630 may be oxide or fused silica layers. Though the waveguide gyrocoil (also referred to as a spiral waveguide) is shown as raised on topof the layer 640, it may be etched into the layer 630 as shown in FIG.4. The input end of the waveguide gyro coil that receives an opticalsignal is denoted as 660, wherein the output end is denoted as 670. Thewaveguide gyro coil has a bottom portion 650 that spirals inwards to thetapered tip 655, where it couples up to the top layer 695 that has therest of the waveguide gyro coil (top portion 699). Though the waveguideportion 699 is shown as raised above the layer 695, it may be etchedinto layer 690. Thickness of a layer 690 (typically an oxide layer inbetween the layers 640 and 695) sets the coupling gap. The top portion699 of waveguide gyro coil starts from the tapered tip 675, and spiralsoutwards to the other tapered tip 680, from where light couples down tothe tapered tip 685 of the waveguide on the bottom plane to go out viaoutput port 670 (to a detector or other optical system components in thefront-end chip). The arrowed dashed lines show the coupling up andcoupling down between the tapered tips in the two planes. The taperdesign and the vertical separation between the two waveguide layersdictate coupling efficiency between the two planes. In order for lightto couple between the two vertically separated waveguide layers, thetapered tips 655 and 675 must have some overlap, and the tapered tips680 and 685 must have some overlap.

FIG. 7 shows a cross sectional view of a structure 700 where input end760 and output end 770 of a waveguide gyro coil are shown, but forclarity, cross section of the intermediate turns of the gyro coil arenot shown. Light couples up to the upper layer waveguide 775 (at thecenter of a coil) and then couples down from the upper layer waveguide780 to the lower layer waveguide to eventually come out from the outputend 770. Layers 710, 720, 730, 740 and 790 may be equivalent to layers610, 620, 630, 640, and 690 shown in FIG. 6. Though not specificallyshown in FIG. 6, the upper layer waveguides (e.g., 699) may have acladding layer, somewhat similar to cladding layer 795 shown in FIG. 7.

It is to be noted that in FIG. 6, the waveguide gyro coil is distributedbetween two planes. However, with advance wafer bonding techniquesdescribed herein, the waveguide gyro coil may be distributed betweenthree or more vertical planes.

FIGS. 8 and 9 show two configuration of an alternative SiPhOG embodimentusing ring resonator waveguides instead of waveguide gyro coil shown inFIG. 6. In the configuration 800 shown in FIG. 8, input light signal 825comes in from the input end of the waveguide 820, couples into the ring810 within the coupling region 822, propagates within the ring resonator810, couples back into the output waveguide 830 within the couplingregion 832, and comes out as output signal 835 via the output end.

In the configuration 900 shown in FIG. 9, input light signal 925 comesin from the input end of the waveguide 920, couples into the ring 910within the coupling region 922, propagates within the ring resonator910, couples back into the output waveguide (same waveguide 920) withinthe coupling region 922, and comes out as output signal 935 via theoutput end.

Note that ring resonators may be formed on a single plane where opticalcoupling happens laterally between the ring and the straight waveguides,as shown in FIGS. 8 and 9. Alternatively, the ring resonator may be on aplane different than the plane of the input and/or output waveguides,requiring optical coupling in the vertical direction.

Persons skilled in the art will appreciate that different generations ofSiPhOGs may be based on interferometric waveguide gyro coils or ringresonators, but both configurations depend on low-loss waveguides andcladding structures as described in FIGS. 1A-5 in this disclosure.

Additionally, the shape of the waveguide-based gyro coil does not haveto be a circular spiral. As shown in FIGS. 10-13, a gyro coil may have arectangular shape (a square being a special kind of rectangle with sidesof equal length). Other shapes, such as elliptical or oval shape arealso possible. In a mask layout, some portions of the layout may havegyro coil waveguides, and other portions may have ring waveguides orother test structures based on waveguides. In addition one may stitch orprint multiple dies (e.g., 2-4 or more dies) together to create onelarger die. The gyro coils (rectangular, circular, or oval or any othershape) may have varying number of turns and varying diameters andvarying shapes of input/output waveguides. Non-circular (e.g.,rectangular) gyro coil designs may provide better utilization of thereal-estate on the mask layout that translates to better utilization ofthe real-estate on the fabricated SiPhOG chip. Note that, in FIGS.10-13, only a few turns of the coils are shown for clarity, where a gyrocoil actually comprises many turns (e.g. hundreds of turns) of closelyspaced waveguides of (e.g., 2.5-3 μm range width per waveguide). Thespacing between adjacent waveguide turns in the coil may be dictated bycross-talk and interference. In summary, the waveguide-based gyros canhave varying geometries as long as the expected performance is harnessedfrom them.

Gyro coils may be fabricated in a single layer or may be distributed inmultiple layers. For a single-layer configuration, the output waveguidehas to cross the turns of the waveguide gyro coil. For example, in thetop view of the embodiment shown in FIG. 10, an optical signal entersthe input end 1030 of the waveguide-based gyro coil 1010 on a SiPhOGchip 1005. The optical signal comes out of the output end 1040 of theoutput waveguide 1035 after propagating through the turns 1020 of thegyro coil 1010. In this example, the pitches p1, p2, p3 and p4 betweenthe waveguide turns at the right, top, left and bottom side of the gyrocoil 1010 are all the same. Therefore this gyro coil is a symmetriccoil. The overlap region between the waveguide turns 1020 and the outputwaveguide 1035 (e.g. the region 1045 showed within the dashed circle)has enhanced loss due to waveguide crossing. The designs shown in FIGS.11 and 13 addresses this problem by making changes in the design of thegyro coil.

In the top view of the embodiment shown in FIG. 11, an optical signalenters the input end 1130 of the waveguide gyro coil 1110 on a SiPhOGchip 1105. The optical signal comes out of the output end 1140 of theoutput waveguide 1135 after propagating through the turns 1120 of thegyro coil 1110. In this example, the pitches p1, p2, p3 and p4 betweenthe waveguide turns at the right, top, left and bottom side of the gyrocoil 1010 are not necessarily the same. In one example, pitches p1, p2and p4 may be the same, but p3 is greater than p1 to accommodateadiabatic taper (e.g., 1205 shown in FIG. 12). Therefore this gyro coilis an asymmetric coil. The overlap region between the waveguide turns1120 and the output waveguide 1135 (e.g. the region 1145 showed withinthe dashed circle, and shown in greater detail in FIG. 12) compensatesfor loss that otherwise would have been there due to waveguide crossingas in FIG. 10.

FIG. 12 shows the adiabatic taper region 1205 at the waveguide crossing.Note that though the taper lengths L1 and L2 are shown to be equal,depending on the waveguide dimension w1, w2, and amount of loss to becompensated, lengths L1 and L2 may be varied. Length of the taper alsoaffects the pitch p3. Larger p3 means more real-estate is utilized onthe mask layout for the gyro coil. The space on the mask layout boundedby the innermost turns of the waveguide gyro coil 1110 may be utilizedto have other test structures on the wafer.

FIG. 13 is an exploded perspective view of a spiral waveguide basedSiPhOG 1300 where the output waveguide does not intersect with the turnsof the gyro coil, and hence avoids enhanced loss due to intersectingwaveguides. This multi-layer design does not need to introduce adiabatictapers (as shown in FIGS. 11-12) either. In the example embodiment shownin FIG. 13, the waveguide turns are on the bottom plane, and the outputwaveguide end 1370 comes out from the top plane in a direction differentfrom the direction of the input waveguide, though the output waveguideend 1370 may then be reoriented in the direction of input end 1360 awayfrom the coil footprint and the light may be coupled back to the bottomplane for efficient coupling. In a preferred embodiment, the outputwaveguide and the input waveguide are to be on the same plane, andportions of the gyro coil may be distributed between the upper and lowerplanes, as shown in FIG. 6 (the only difference would be the shape ofthe spiral is rectangular instead of circular shown in FIG. 6). Theoutput waveguides could be in same side of the chip or on another sideof chip as the input waveguide. In one embodiment, light couples up tothe upper plane and then couples back down to the lower plane, if boththe input and output waveguides are in the lower plane. In anotherembodiment, light coupled down to the lower plane and then couples backup to the upper plane, if both the input and output waveguides are inthe upper plane. Note that though the example embodiments are describedwith two planes, there may be more than two planes, each having portionsof the gyro coil, and light being coupled up and down (without changingdirections) between the multiple planes.

In FIG. 13, a substrate layer and some of the other interposing layersare not shown for clarity. The layer 1340 with the gyro coil 1350 ispreferably fused silica. The input end that receives an optical signalis denoted as 1360, wherein the output end is denoted as 1370. The gyrocoil 1350 spirals inwards to the tapered tip 1355, where it couples upto the top layer 1395 that has output waveguide 1399. Thickness of alayer 1390 (typically an oxide layer in between the layers 1340 and1395) sets the coupling gap. The output waveguide starts from thetapered tip 1375, and bends outwards to the output end 1370 to adetector or other optical system components. The arrowed dashed lineshow the coupling up between the tapered tips in the two planes. Thetaper design and the vertical separation between the two layers withwaveguides dictate coupling efficiency between the two planes. In orderfor light to couple between the two vertical planes, the tapered tips1355 and 1375 must have some overlap determined by the taper design.

In summary, the present disclosure describes various embodiments forsingle-layer and/or multi-layer designs of low-loss waveguide basedgyroscope coils for a SiPhOG system. The propagation loss in the SiNwaveguides described herein can be well below 0.1 db/meter. This is avast improvement over the current state-of-the-art SiN process withpropagation loss in the range of 0.1 db/centimeter. The key to loweringthe loss while using the standard silicon fab processes and equipment isto use high quality fused silica (sometimes called “glass”) wafers,and/or, using wafers with an oxide layer serving as part of the claddingaround a core. A repeated deposition/anneal steps during oxidedeposition ensure precise control of thickness of the oxide layer, aswell as expunging trapped hydrogen from the deposited sub-layers.

In the foregoing specification, implementations of the disclosure havebeen described with reference to specific example implementationsthereof. It will be evident that various modifications may be madethereto without departing from the broader spirit and scope ofimplementations of the disclosure as set forth in the following claims.The specification and drawings are, accordingly, to be regarded in anillustrative sense rather than a restrictive sense. Additionally, thedirectional terms, e.g., “top”, “bottom” etc. do not restrict the scopeof the disclosure to any fixed orientation, but encompasses variouspermutations and combinations of orientations.

What is claimed is:
 1. A waveguide structure, comprising: a first fusedsilica wafer with a first etched trench thereon, wherein the first fusedsilica wafer acts as a first cladding; a first core formed in the firstetched trench of the first fused silica wafer, wherein the first corecomprises silicon nitride (SiN); an interposing layer that separates twoplanes of the waveguide structure; a second fused silica wafer with asecond trench thereon, wherein the second fused silica wafer acts as asecond cladding; and a second core formed in the second etched trench ofthe second fused silica wafer, wherein the second core comprises siliconnitride (SiN), wherein the first fused silica wafer with the first coreis bonded to a first surface of the interposing layer, and the secondfused silica wafer with the second core is bonded to a second surface ofthe interposing layer, wherein the first surface and the second surfaceof the interposing layer are opposite to one another.
 2. The waveguidestructure of claim 1, wherein the first core is completely surrounded bythe first fused silica wafer and the interposing layer bonded thereonwith the first surface of the interposing layer adjacent to the firstcore.
 3. The waveguide structure of claim 2, wherein the second fusedsilica wafer with the second core is flipped and bonded to theinterposing layer such that the second core is completely surrounded bythe second fused silica wafer and the interposing layer with the secondsurface of the interposing layer adjacent to the second core.
 4. Thewaveguide structure of claim 1, wherein dimensions and shape of thefirst etched trench are designed according to target dimensions andshape of the first core, and dimensions and shape of the second etchedtrench are designed according to target dimensions and shape of thesecond core.
 5. The waveguide structure of claim 1, wherein a refractiveindex of the interposing layer is substantially the same or identical toa refractive index of fused silica.
 6. The waveguide structure of claim1, wherein the interposing layer comprises an oxide layer formed on thefirst fused silica wafer.
 7. The waveguide structure of claim 6, whereinthe oxide layer is formed using a silane-based precursor, achlorine-based precursor, or a deuterated source.
 8. The waveguidestructure of claim 6, wherein the oxide layer is formed by depositingmultiple sub-layers of the oxide.
 9. The waveguide structure of claim 8,wherein before depositing each sub-layer of the oxide, a respectiveannealing step is performed to drive out excess hydrogen.
 10. Thewaveguide structure of claim 9, wherein the annealing temperature isgreater than 1100 degrees Celcius and the annealing duration is greaterthan or equal to 2 hours.
 11. The waveguide structure of claim 3,wherein the interposing layer comprises a third fused silica waferpolished down to a target thickness to vertically separate the firstcore and the second core.
 12. The waveguide structure of claim 1,wherein bonding between the first fused silica wafer and the interposinglayer and bonding between the second fused silica wafer and theinterposing layer are done by activation of bonding surfaces by oxygenplasma followed by a thermo-mechanical process.
 13. The waveguidestructure of claim 1, wherein bonding between the first fused silicawafer and the interposing layer and bonding between the second fusedsilica wafer and the interposing layer are done by anodic bonding. 14.The waveguide structure of claim 1, wherein the waveguide structure ispart of a waveguide chip that is used as a rotation sensing unit of anoptical gyroscope.
 15. The waveguide structure of claim 14, wherein aportion of the waveguide structure is arranged on the waveguide chip asa loop comprising a plurality of turns forming a coil that acts as therotation sensing unit of the optical gyroscope.
 16. The waveguidestructure of claim 15, wherein a first tapered end of a portion of thecoil on a first plane vertically overlaps with a corresponding secondtapered end of a second portion of the coil in a second plane that isvertically separated from the first plane via the interposing layer,wherein a direction of propagation of light in the first portion and thesecond portion of the coil remains unchanged.
 17. The waveguide of claim15, wherein light is evanescently coupled between the first tapered endon the first plane and the second tapered end on the second plane.
 18. Awaveguide structure, comprising: a first fused silica wafer acting as afirst cladding; a patterned silicon nitride (SiN) layer formed on top ofthe first fused silica wafer, wherein the patterned SiN layer acts as afirst core; an oxide layer coinciding with the patterned SiN layer,wherein the oxide layer surrounds the first core from the sides; aninterposing layer that separates two planes of the waveguide structure;a second fused silica wafer with a trench thereon, wherein the secondfused silica wafer acts as a second cladding; a second core formed inthe etched trench of the second fused silica wafer, wherein the secondcore comprises silicon nitride (SiN), wherein the first fused silicawafer with the oxide layer and the first core formed thereon is bondedto a first surface of the interposing layer, and the second fused silicawafer with the second core is bonded to a second surface of theinterposing layer, wherein the first surface and the second surface ofthe interposing layer are opposite to one another.
 19. The waveguidestructure of claim 18, wherein an initial deposited thickness of theoxide layer is reduced by chemical mechanical polishing (CMP), such thatso that a top surface of the oxide layer is substantially planar andflush with a top surface of the SiN layer acting as the first core.